USSR Patent No. 28 31 85 describes catalysts composed of nickel, aluminum oxide, and zirconium dioxide. These catalysts are prepared by precipitating nickel and aluminum oxide onto zirconium dioxide, which constitutes the support.
Nickel-zirconium dioxide catalysts are described in U.S. Pat. No. 2,564,331. Therein, an aqueous solution of zirconium sulfate is added to an aqueous solution of sodium carbonate, the latter being present in large excess. A basic zirconium compound is precipitated which redissolves due to the excess of carbonate ions. Thereafter, aqueous nickel sulfate is added at about 74.degree. C. and a mixed precipitate consisting of nickel-zirconium carbonate is formed. This is further processed by washing, drying, calcination, and reduction.
A process for the preparation of catalysts by simultaneous precipitation of metals in the form of their insoluble oxalates is taught in U.S. Pat. No. 3,876,557. The pH value is maintained at 1.0 to 3.0 and a wide variety of metals are indicated as being useful. Specifically, nickel, cobalt, iron, copper, zinc, zirconium, aluminum, barium, calcium, strontium, and magnesium are all named specifically.
DE AS 12 57 753 sets forth the preparation of zirconium dioxide-activated nickel catalyst which is formed by the precipitation of insoluble carbonates. Ammonia and carbon dioxide are evaporated from a solution of ammonium zirconyl carbonate and nickel ammine carbonate sufficiently to ensure that a mixture of basic carbonates precipitates out at about 82.degree. C. or higher. After filtration, the mixture is dried, calcined and reduced.
When catalyst compositions consisting of two or three components are prepared by precipitation, care must be taken to make sure that the precipitation leads to as homogeneous a mixture of the individual components as possible. The afore-mentioned processes do not give sufficient guarantee of this.
According to the procedure described in the US-PS 2,564,331 a large excess of carbonate is used. This means that initially pure nickel carbonate is precipitated and the actual mixed precipitation only takes place within a narrow range whilst towards the end of the precipitation only zirconium carbonate is deposited.
If the processes described in the US-PS 3,867,557 and the DE-AS 12 57 753 are followed, a gradual shifting of the pH value during precipitation is achieved, on the one hand, by adding an aqueous oxalic acid solution to a metal mixed-salt solution and, on the other hand, by evaporating ammonia and carbon dioxide.
As the solubility of the individual components greatly depends on the prevailing pH value, the mixture which is precipitated always correlates with the pH value. As the precipitated mixture changes its composition as a function of the pH value, differently composed, inhomogeneous precipitated mixtures are obtained due to the change in the pH value.
As these difficulties already occur when only two components--e.g. nickel and zirconium compounds--are precipitated, it must be assumed that precipitation of homogeneous coprecipitates consisting of three metal compounds must fulfil special requirements. It is the purpose of the present invention to solve this problem.